It is known, in particular from European patent application no. 0 336 337, to construct a superconductive limiter by using a coil placed in a cryostat, the coil comprising two coaxial windings of superconductive material wound in opposite directions.
In order to increase the power of the limiter, it has been suggested that a coil assembly may be constructed from a plurality of coils of the aforementioned type physically disposed coaxially and electrically connected in series.
The problem arises in such a coil assembly of providing insulation between each of the windings, for each of the constituent coils.
If the windings are to be insulated from one another, it is necessary to use an insulating layer capable of withstanding a very high voltage. Assuming that one of the windings has a region in which the superconductive state disappears to be replaced by the normal state (this region is henceforth designated as the transition region), then a significant electric voltage is developed between the transition region and the adjacent region of the other winding. If there is insufficient insulation, the voltage may give rise to a breakdown between windings. However, good insulation presupposes a thick layer which will increase significantly the volume occupied by the coil assembly. Moreover, this electrically insulating layer also acts as a heat insulator, so that the heat generated in the region of the winding which is in transition cannot easily be transmitted to the adjacent region of the other winding, whose own transition may lag behind.